A continuing goal of electrical engineers has been the goal of reducing the physical size and parts count of switchmode power converters. The blending together or combining of inductors and transformers into single physical assemblies with little or no compromise in the desired conversion characteristics has been termed "integrated magnetics". One important reason for this effort has been the fact that inductors and transformers are major contributors to the total cost, weight and size of a converter system. Magnetic integration, if properly executed in the design of power converters, can bring added benefits in electrical performance, such as reduced stress on the components.
An excellent overview of the state of the art is presented in Chapter 12 of the book "Modern DC-to-DC Switchmode Power Converter Circuits," by Rudolf P. Severns and Gordon E. Bloom, van Nostrand Reinhold Company, 1985. A comprehensive background of the subject is contained in the two volume book "Modern DC-to-DC Switched-Mode Power Conversion," by R. D. Middlebrook and Slobodan Cuk; TESLAco, Pasadena, Calif. (1983). Some U.S. patents on the subject of integrated magnetics and converters are:
U.S. Pat. Nos. 4,675,797 4,675,796 4,538,219 4,257,087 3,694,726 4,688,160 4,561,046 4,355,352 4,262,328 3,553,620
The process of magnetic integration is not simple. For the most part, integrating magnetic elements of a converter does not add to the many design aspects and difficulties with which an engineer must contend during the converter's development phase. However, it does place additional burden on the designer to properly specify the performance of the integrated magnetic elements and a burden on the manufacturer to insure a consistent product which can be more complex than a simple transformer or inductor assembly. For example, there are some subtle electrical problems which arise when coupling inductors together on a common magnetic core. Imbalances in the required turns ratio relationships between the inductor and transformer winding can produce circulating currents in filter networks, resulting in excessive ripple current. High power loss in filter capacitors can result in converter damage and even destruction of the filter elements. Thus, integrated magnetics, as applied to switchmode power converter circuits, is a concept which is not straight-forward or easy to apply in practice.
Any advancement that can be made and any teaching in this highly complex subject area will be welcome by those skilled in the art, and any practical embodiment of these teachings will advance the state of the art. This is particularly true if subtle design considerations are highlighted and circuits are presented which result in unexpected improved performance or operating characteristics, especially when only a few basic components are used to create a product which can be produced in quantity at a competitive price.